The present invention relates to the superplastic forming of metal parts, and particularly to a method of removing the same from a die cavity in which the parts are superplastically formed.
U.S. Pat. Nos. 4,381,657 and 4,502,309 to Hamilton et al discuss the problems attendant with removing superplastically formed parts from die cavities. As discussed, the problem lies in the fact that ductile metals are formed at high temperatures and thus under low stress conditions, and are removed while they are still at a substantially high temperature. Because of this, the parts are still relatively soft and thus have a low strength characteristic such that they are easily deformed in the process of being removed. If the part is allowed to cool, the die will also cool making production rates exceedingly low as well as requiring substantial energy to reheat the die. In addition, differential shrinkage between the part and the die can lead to part deformation as well as locking of the part in the die, particularly with dies having near zero draft angles for the die cavity.
In addition, because of the high temperatures at which superplasticity is effected, lubricants are ordinarily not effective; many lubricants evaporate or combust at such temperatures. Hence, when the part is formed between the dies, there is intimate metal-to-metal contact.
Boron nitride is a commonly used lubricant/separating agent. It, however, does not prevent sticking and presents other problems discussed below.
Both of the above patents disclose a vacuum method of removing a superplastically formed part from a die cavity. More particularly, a vacuum is formed between an upper die and the formed part in a cavity of the lower die. The vacuum secures the part to the face of the upper die. The dies are then separated, the upper die pulling the part from the cavity of the lower die.
In pulling a part from a die cavity there remains the possibility of deforming and stretching the part if the part in the cavity tends to stick to the surfaces of the cavity. In addition, when a vacuum is employed it is often pulled through small openings in a die structure, which can be plugged by scale and any lubricant that may be used. The above-mentioned boron nitride, for example, is a slurry formulation; the liquid portion thereof evaporates at the elevated temperatures employed for superplastic forming leaving a powder residue on the surfaces of the dies.